I am in a somewhat similar situation, in that my wife was always a different person around her parents. I've come to realize she always saw herself as their daughter first, and my spouse third (being a mother was second).
There's not much you can do when they have an enabler or someone like that who causes shifts in behavior. Fix yourself first - that's what I'm trying to do. It's helping - I'm actually calmer and more relaxed despite this crazy divorce business.
Me: 35 W: 31 D10, S7, S2, S11 months M: 11 years Tricked into separation. In Last Resort.
My story: http://www.divorcebusting.com/forums/ubbthreads.php?ubb=showflat&Number=1800530#Post180053